This invention relates to ringer generator circuits and more particularly to a ringer-power generator circuit for the carrier subscriber terminal of a carrier telephone communication system.
Single and multichannel subscriber carrier telephone systems are finding increased application since they may provide additional communication channels over existing cable pairs. Carrier subscriber terminals of such systems may be energized directly by power from the central office on the cable pair, by local power at the subscriber's location, or by a local battery that is charged by central office battery current on the cable pair. Some subscriber carrier telephone systems and associated carrier subscriber terminal ringer generator circuits are generally described in the prior-art U.S. Pat. Nos. 3,886,321, May 27, 1975 by Krasin et al; 3,904,833, Sept. 9, 1975 by Beene; and 3,927,266, Dec. 16, 1975 by Stewart et al. In this Krasin patent, a 300 Hz central office ringing signal, which is detected at the subscriber terminal, keys a 20 Hz oscillator. Energization of the oscillator is delayed to prevent ringing on transients. The oscillator logically drives a switch which alternately connects a unidirectional DC voltage of +225 volts and 0 volts to one of the tip and ring leads of a handset to energize the ringer thereof. Experience has shown that it is undesirable to have such a high DC voltage present on traces of a printed circuit board since it may damage them. Employing a unidirectional ringing voltage also requires additional set-up time for an installer to polarize a ringer prior to connection of the associated handset to the tip and ring lines. Krasin discloses a subscriber carrier system with another ringer circuit in his U.S. Pat. No. 3,510,584, dated May 5, 1970. The Beene patent discloses a system in which the subscriber terminal is powered from the central office. A stepped up line voltage from the central office during ringing causes a circuit in the subscriber terminal to energize a blocking oscillator of a DC-DC power converter to produce constant amplitude pulses of a prescribed frequency for charging a capacitor. The converter includes a Zener diode which senses the output voltage on the capacitor and limits it to a maximum level. A central office ringing signal is also detected to produce a constant voltage pulse throughout a ringing period which turns on a squarewave generator. The output of the generator causes a pair of transistors to alternately conduct to connect the capacitor to and disconnect it from the ringer of a handset. This method of generating a local subscriber ringing signal for driving a handset ringer is relatively complex. The invention in this patent is an improvement over that in the Stewart U.S. Pat. No. 3,927,266. The Stewart patent discloses a subscriber carrier system in which the subscriber terminal includes a local battery which may be charged from the central office battery and a handset that requires only a two-wire drop. The subscriber ringer circuit there employs a constant voltage type DC-to-DC voltage converter which is powered only during the ringing period of a ringing cycle to simultaneously produce both positive and negative DC ringing voltages that are alternately connected to one of the tip and ring lines. It has been found that such a converter may be short circuit unstable which makes ring trip difficult to accomplish. Such a converter circuit also employs two transistors as well as a series switch to turn it on and off, the latter reducing the circuit efficiency.
An object of this invention is the provision of an improved ringer generator circuit for the carrier subscriber terminal of a subscriber carrier telephone system.